General Article

International Journal of Sustainable Building Technology and Urban Development. 30 December 2023. 437-454
https://doi.org/10.22712/susb.20230034

ABSTRACT


MAIN

  • Introduction

  •   Research Significance

  • Material Properties

  • Beam Specimen Details

  • Experimental Test Setup and Instrumentation

  • Microstructure of GP Concrete Samples

  • Results and Discussions

  •   Properties of Fresh Concrete

  •   Mechanical Properties of Concrete

  •   Durability Performance of Concrete

  •   Load-deflection Behaviour of Beams

  •   Service Stiffness

  •   Post-peak Stiffness

  •   Flexural Toughness

  •   Ductility Index

  •   Longitudinal Strain Profile of Under-reinforced Concrete Beams

  •   Moment Curvature Relationship of Under-reinforced Concrete Beams

  •   SEM Micrographs of GP Concrete Samples

  • Conclusion

Introduction

The surge in concrete demand led to a substantial rise in cement production in India, reaching 298 million tonne in 2022, and representing 8% of the global installed cement capacity [1], which exacerbates the environmental impact in terms of carbon emissions. India is indeed the second largest cement producer globally [2], comprising about 250 large cement plants [1, 3], and hence has a significant impact on global warming effect due to the carbon emissions [4, 5, 6, 7]. The onset of changes in precipitation patterns which led to drought and flood in India can be perceived through global warming effect according to the previous studies [8, 9]. Since the cement manufacturing units emits a comparable volume of CO2 to the atmosphere [10, 11, 12, 13, 14], it is indispensable to explore partial alternatives to traditional cement in the construction sector, in view of promoting a carbon-neutral atmosphere and cleaner production. The concept, cleaner production, put forward by the United Nations Environmental Programme (UNEP) in 1989 [15] is of utmost importance for a densely populated and developing country like India, as it encompasses eco-efficiency, pollution prevention and green productivity [16, 17]. One of the Environmental Performance Index (EPI) indicators considered in the present study is air pollution, viz. through CO2 emissions, released mostly from the cement plants in India [18]. However, India is on the verge of fostering a green environment by focusing on smart cities and sustainable initiatives [19]. Correspondingly, the country has joined the 2030 agenda for sustainable development with the aim of creating a safer and more prosperous planet [20]. Recognising the significance of developing sustainable strategies, the government of India allocates funds to cleaner production dissemination programmes [20, 21]. National cleaner production centres (NCPC) set out by the UNEP and the United Nations Industrial Development Organisation (UNIDO) that came into existence since 1993 in India promote sustainable development [22].

Focusing on low carbon and circular economy in the concrete sector, many past research works recognised the use of supplementary binders such as fly ash, ground granulated blast furnace slag (GGBFS), silica fume, metakaolin, glass powder (GP) and so on [18, 23, 24, 25]. In recent times, the diminishing number of coal-fired power stations and the reduction in blast furnace operations will lead to a scarce supply of fly ash and GGBFS respectively. Limited economic feasibility of SCMs like silica fume and metakaoline is attributed to its high material cost. However, non-degradable glass is found abundantly as a landfill in India. Only 45% of the total glass waste produced in 2019 had been recycled, which accounts for 21 billion tonnes; and the one that highlights the urgent requirement for the management of glass waste in the country [26]. The fact that glass takes 4000 years to decompose is quite alarming [27, 28]. It is worth noting that recycling a single ton of glass has the potential to conserve one ton of natural resources; including 19 litres of oil, 42 kWh of energy, prevent the release of 3.4 kg of pollutants and save 1.5 cubic metres of landfill space [29]. In addition, the substitution of glass powder (GP) as a cementing material promotes sustainability through several strategies like reducing waste generation, lowering cement consumption and associated CO2 emissions, conservation of energy, enhancing durability and minimising resource depletion.

Comprehensive research has been done on fresh, hardened and durability properties of GP concrete [30, 31, 32, 33, 34, 35, 36], still extensive experimental studies on analysing the flexural behaviour of GP concrete are limited. Hence the current work intends in analysing the flexural parameters of GP concrete beams with varying longitudinal reinforcing ratio. Reinforced concrete beams play a vital role in transmitting the gravitational and environmental loads uniformly from the structure to the supporting ground and limiting the occurrence of internal stresses, strains, and deflections. Therefore, load-deflection behaviour, crack pattern, failure mode and moment curvature relationship of the structural elements cast with partial substitution of cement by GP must be investigated experimentally before its implementation in the construction field.

The present study includes the assessment of the flexural behaviour of concrete beams with GP content and considering the longitudinal reinforcement ratio as the main test parameter. A brief outline of the experimental programme is illustrated in Figure 1. The optimal cement replacement (30%) with GP having two different particle sizes as obtained from the hardened properties is considered as the GP substitution for the present study [32]. The percentage of reinforcement considered for studying the flexural behaviour is 1.92% for under-reinforced sections and 2.88% for over-reinforced sections. For under-reinforced sections, it is a well-known fact that yielding of reinforcement takes place initially and thereafter failure occur by the crushing of concrete. Howbeit, to know the precise character of GP infused concrete beams, over-reinforced concrete sections are experimentally investigated so that no yielding of steel happens, and failure load depends purely upon the concrete strength. The resistance of the concrete beam to deformation during the transition from elastic to plastic zone under loads till failure is evaluated in terms of flexural toughness, displacement ductility and stiffness factors as determined from the load-deflection profile. Further, analysis on the longitudinal concrete strains and moment curvature of the under-reinforced beams are done to assess the capability of the GP beams. Comparison of the GP beams with the respective control beams (GP0) for both under-reinforced and over-reinforced sections is also carried out.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F1.jpg
Figure 1.

Brief outline of the current work.

Research Significance

An investigation into the structural behaviour of glass powder (GP) concrete has become significant due to its sustainability as a cementing material in concrete. By adding GP as a partial replacement for cement, cement consumption has been reduced, which consequently lowers CO2 emissions. In this context, an eco-friendly environment is aimed at by keeping out of the non-decomposable glass from landfills and preservation of natural resources. The authors’ prior research (Paul et al., 2022) has shown that incorporating GP to concrete enhances its mechanical and durability properties, and hence is a sustainable and circular solution. The microstructural analysis on GP concrete samples were conducted to understand the behaviour of its interface. Further investigation into the impact of GP on concrete structures is imperative before it can be widely used, especially studying its flexural behaviour. The results of this experimental research offer reliable information for numerical modelling to evaluate the flexural behaviour of reinforced GP concrete beams.

Material Properties

The materials used for the present study include: ordinary Portland cement 53 grade according to IS 269:2015 [37]; two GP used as cement replacement in equal proportions (GP1 and GP2 with mean particle sizes of 31.57 µm and 7.8 µm, respectively); superplasticiser having a density of 1200 kg/m3; potable water; manufacturing sand (M-sand) passing through a 4.75 mm sieve and a density of 2600 kg/m3 as fine aggregates and 20 mm nominal size having a density 2700 kg/m3 as coarse aggregates, complying with IS 383:2016 [38]. Workability of the fresh concrete mix was assessed by slump cone test in accordance with ASTM C143M-03 [39]. Mix proportioning was carried out based on IS standards [40, 41] and the mix proportion arrived is 1:1.8:3.2 for M40 grade concrete and is presented in Table 1.

After assessing the mechanical properties of concrete with varying GP substitutions (0%, 10%, 20%, 30% and 40%), the durability performance of GP concrete like sorptivity and rapid chloride-ion permeability test (RCPT) with the same GP substitution levels was determined. Sorptivity was assessed according to ASTM C1585-04:2007 [42] while the resistance to chloride-ion penetration was carried out following the rapid chloride-ion permeability test (RCPT) according to ASTM C1202:2012 [43]. Three cylindrical specimens, 100 mm in diameter and 50 mm in height, were prepared for each concrete mix proportion as given in Table 1. The horizontally sliced cylinders with 50 mm thickness and 100 mm diameter were dried at 50°C in a dry oven until a constant mass was achieved. Then, the specimens were sealed using a silicone sealant gel on the curved surface to ensure a uniform water entrance by capillarity allowing only a one-dimensional flow from the bottom surface. After taking the weight of the sealed specimens, the specimens were immersed in water to a depth of 5 to 10 mm through fibre rods. The specimens were weighed at 1, 5, 10, 20, 30, and 60 minutes, then at each hour up to six hours, and finally, at every 24 hours up to eight days. The unidirectional water absorption (I) and the sorptivity coefficient (S) were calculated using Equations (1) and (2).

(1)
I=Δma×d
(2)
S=It

where, I - cumulative water absorption of the tested specimen in mm

Δm - increase in mass of the tested specimen in grams

a - contact area in mm2

d - density of water - 0.001 g/mm3

S - sorptivity coefficient in mm/s0.5

t - time of exposure in s

Table 1.

Mix proportion of M40 grade concrete

Concrete
designation
Cement GP Fine aggregate Coarse
aggregate
Superplasticiser w/b
ratio
GP1 GP2
(IS 10262:2019, IS 456:2016) kg/m3
M40 GP0 368.80 0 0 663.70 1163.4 1.84 0.40
M40 GP10 331.92 18.44 18.44 663.70 1163.4 1.84 0.40
M40 GP20 295.04 36.88 36.88 663.70 1163.4 1.84 0.40
M40 GP30 258.16 55.32 55.32 663.70 1163.4 1.84 0.40
M40 GP40 221.28 73.76 73.76 663.70 1163.4 1.84 0.40

The specimen was placed in RCPT apparatus such that one face is exposed to 3% of NaCl solution, and the other face is exposed to 0.3 N NaOH solution. A potential difference of 60 V DC was maintained across the ends of the specimen. The current between the electrodes was measured every 30 minutes for 6 hours. The total charge passed through each specimen represents the chloride-ion penetration of concrete and is calculated using the Equation (3).

(3)
Q=900(I0+I360+2(I30+I60+I90++I330))

where I0 is the current noted immediately after the voltage is applied and I360 is the current noted after 360 minutes of testing. I30, I60…..I330 are the intermediate current in Ampere pointed out at every 30 minutes interval.

Reinforcement detailing of beams was done using deformed steel bars having a grade of 500 N/mm2. Details of the test bars meant for the longitudinal and transverse reinforcement in beams are presented in Table 2.

Table 2.

Mechanical properties of reinforcing steel

Bar diameter (mm) 6 8 12
Yield strength (N/mm2) 485.49 546.18 563.52
Ultimate tensile strength (N/mm2) 624.20 604.70 641.54
Elastic modulus (N/mm2) 200282 202049 201611
Elongation (%) 22.08 21.57 17.24
Reduction in area (%) 21.67 28.75 36.25

Beam Specimen Details

Eight beams were designed following IS 456:2005 [41] standards as singly reinforced with a depth-to-width ratio of 1.5. The total span to effective depth ratio was kept below twenty to reduce shear stress and to limit deflection, according to IS specification. Four beams were designed as under-reinforced (1.92%) and four as over-reinforced (2.88%) to study the flexural behaviour of M40 concrete with 30% hybrid GP and without GP (normal beams). Two specimens were cast for each case. The flexure beams were labelled with “FL” or “FH” in the case of under-reinforced (lightly-reinforced) or over-reinforced (heavily-reinforced) concrete sections, respectively and the two sets of specimens were represented as “1” and “2” for each case.

The flexural behaviour was evaluated under pure bending theory, where longitudinal strains are proportional to the distance from the neutral axis. The failure pattern of concrete for under-reinforced beams subjected to flexure cannot be determined precisely, as yielding of reinforcement is expected to occur before crushing of concrete. Hence, over-reinforced beams with same dimensions were also tested to predict more accurately the behaviour of GP concrete. A comparative study of behaviour of GP concrete beam with conventional concrete beam have also been conducted for both under and over-reinforced beams. The flexural behaviour of GP concrete beam is compared with the behaviour of the counterpart normal beams (GP0). Top of Form.

The dimensions and reinforcement details of beams for flexure for different longitudinal ratios is as shown in Figure 2 and a sample of the reinforcement cage is shown in Figure 3.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F2.jpg
Figure 2.

Dimensions and reinforcement detailing of beams designed for flexure.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F3.jpg
Figure 3.

Reinforcement cage for casting beams for flexure.

Experimental Test Setup and Instrumentation

A four-point bending test was performed on the concrete beam specimens in a Universal Testing Machine (UTM) having 3000 kN capacity under load control conditions at a constant loading rate. The schematic diagram of the automated test setup for analysing the flexural behaviour is shown in Figure 4. Three LVDTs (Linear Variable Displacement Transducer) of 20 mm gauge length with a least count of 0.01 mm were positioned one at the midspan, and two at the loaded positions to measure the vertical displacement under load application until the failure of beams. Buttons of DEMEC mechanical gauge (gauge length 10 cm and a least count of 0.002 mm) were fixed on the lateral surface of the beam to measure the strains at load position and at midspan along the top and bottom level of reinforcements. Application of load was done by a load cell of 50 tonne (490.5 kN) capacity with a least count of 0.1 tonne (0.981 kN) placed at the top of the test specimen and the applied load was displayed on the load indicator. The load was transferred from the actuator to the cast iron spreader beam. The total load from the spreader beam was transferred to the two loading points on the beam specimen separated at 350 mm apart at the centre where a pure bending region was developed. The beam supports were fixed symmetrically at a distance of 75 mm from both the ends of the beam. A data acquisition system (DAQ) was attached to record continuously the deflection and strain corresponding to the applied load and a laptop is used to retrieve the recorded data for analysis. The load is applied at an increment of 10 kN until failure of beam and corresponding output were recorded. A crack-detection microscope of range 0 to 3 mm and having a least count of 0.02 mm was used to measure the crack width. The schematic diagram and photograph of the test setup is shown in Figure 4.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F4.jpg
Figure 4.

Beam test setup.

Microstructure of GP Concrete Samples

Behaviour of the interfacial transition zone (ITZ) of the GP concrete samples was identified by performing SEM analysis (TESCAN VEGA 3 USA, Inc.) using a secondary electron (SE) detector. Oven-dried button-shaped test samples of GP concrete with a size of 10 mm were used for the microstructure analysis.

Results and Discussions

The fresh property of the concrete mixes was initially examined followed by the assessment of mechanical properties of concrete with GP at varied cement substitutions to find out the optimal GP substitution based on the cube compressive strength. Subsequently, investigation on durability properties were carried out on concrete specimens with varying GP content. Reinforced concrete beams were cast with the concrete mix that produced highest cube compressive strength. The reinforced concrete beams were evaluated under a four-point bending test to obtain a larger area of stress distribution which led to an easier identification of any defects and flaws and enabled a more realistic calculation of the ductility factor. Results are focused on the influence of hybrid GP in concrete at the material level for the greener production of structural concrete beams.

Properties of Fresh Concrete

Workability of the fresh concrete mix as measured by slump test show an increased workability for GP30 concrete mix than GP0 mix. The slump for GP0 and GP30 mixes were 75 mm and 95 mm respectively. The non-absorbent glass powders (GP1 and GP2) in the GP30 mix have contributed to the easiness of mixing, placing and transporting the GP mix. Owing to the enhancement in the workability of GP30 concrete, the mix GP30 with equivalent amounts of superplasticiser was adequate as that of its counterpart concrete mix without GP.

Mechanical Properties of Concrete

Compressive strengths of 50.22, 48.59, 49.93, 52.74 and 48.15 N/mm2 were obtained at 28 days age for the M40 grade concrete with 0, 10, 20, 30 and 40% GP as cement replacement, respectively. An increase in mechanical strengths were observed up to a cement substitution of 30% GP, followed by a decrease in strengths with 40% GP substitution. The decreased strength is attributed to the unavailability of CH in the mix, with 40% GP, to form the additional CSH structure as the entire CH is consumed during the pozzolanic reaction. Thus, the optimum GP substitution level was determined from the cube compressive strength test results which produced highest compressive strength, ie., at 30% GP substitution. Hence the present investigation considered GP beams with 30% GP content.

Durability Performance of Concrete

Investigations on durability studies like sorptivity and rapid chloride permeability tests were carried out on concrete containing varying GP substitution levels are discussed in this section.

The cumulative capillary water absorbed versus the square root of elapsed time of immersion in water for M40 grade concrete is shown in Figure 5. The initial absorption rate was higher during the first six hours. The sorption trend for the subsequent hours or days occurred at a diminished rate. The lowest sorption was obtained for the specimen with increasing GP content, at 40% GP content.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F5.jpg
Figure 5.

Sorptivity results.

Charge passed and current (in Amperes) versus time (in seconds) for M40 grade concrete specimens with different GP percentage levels is shown in Figure 6(a) and (b) respectively. The resistance to chloride-ion penetration presented a decreasing pattern of current flow with an increase in GP content.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F6.jpg
Figure 6.

RCPT results.

Load-deflection Behaviour of Beams

Beam specimens (GP0 and GP30) of M40 grade concrete were tested for flexure under uniaxial compressive loading until failure. The deflection at the LVDT points and strains were recorded automatically for every load increment. The increase in load up to the ultimate load in under-reinforced beams (FLGP0-1, FLGP0-2, FLGP30-1, FLGP30-2) leads to the yielding of longitudinal reinforcement in the tension zone followed by the compression failure of concrete. This peak load occurred in advance of the failure of the beam by the widening of a single crack, and failure load observed was recorded. In the case of over-reinforced beams (FHGP0-1, FHGP0-2, FHGP30-1, FHGP30-2), failure in concrete occurred by pure compression, and a higher crushing load was experienced by all the beams compared to that of the under-reinforced beams. However, lower deflection was observed in the over-reinforced beams due to the lack of yielding in the tensile zone.

The load-deflection curves obtained from the experimental values for both under and over-reinforced beams as in Figure 7 shows a linear elastic behaviour up to the first crack load, which depicts that the deflection of the beam is proportional to the applied load till the crack initiation. Further increase in load up to failure load show a nonlinear deflection profile and results in the development of discrete flexure cracks along the midspan of the test beams as illustrated in Figure 8. This increased load led to the propagation of additional cracks and the widening of the existing cracks. Flexural cracks are seen in all the beam specimens in the constant moment region. Increased ultimate load carrying capacity of over-reinforced beams (FHGP0-1, FHGP0-2, FHGP30-1, FHGP30-2) is attributed to the increase in the quantity of reinforcement in the tensile zone [44, 45]. Yielding of steel is not present in the over-reinforced sections, whereas crushing of concrete is obvious with the increase in the applied load. But in the case of under-reinforced beams the increase in load after the first crack load led to the yielding of the longitudinal bars in the tensile zone. After a while, the specimen reaches the peak load, followed by the crushing of concrete in the compression zone. Shortly, failure of the beam takes place by a progressive decrease in load with the widening of a particular single crack. Higher ultimate load values and higher deflection values are observed for GP30 specimens compared to the normal specimens for both under and over-reinforced beams.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F7.jpg
Figure 7.

Load-deflection profile for different reinforcement ratios.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F8.jpg
Figure 8.

Photograph of beams with flexure cracks.

First crack moment, yield moment and ultimate moment as determined from the load-deflection diagram corresponding to the deviation of the initial slope, yielding zone and ultimate load respectively are illustrated in Figure 9. Increase in the moments with respect to both GP content and the increasing reinforcing ratio is obvious from Figure 9. Yield moment was not recorded for the over-reinforced beams as it did not experience any yielding.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F9.jpg
Figure 9.

First crack moment, yield moment and ultimate moments of flexure beams.

Test summary of flexure beams presented in Table 3 shows an increase in both load and deflection values with an increase in the hybrid GP content for the under-reinforced beams. Increased load carrying capacities with a decrease in the deflection values than the under-reinforced ones is observed for the over-reinforced beams which is due to the absence of yielding in over-reinforced sections.

Table 3.

Summary of flexure beam test results

Beam designation First crack Yield Ultimate Failure (80% drop)
Load
(kN)
deflection
(mm)
Load
(kN)
deflection
(mm)
Load
(kN)
deflection
(mm)
Load
(kN)
deflection (mm)
FLGP0-1 17.3 0.4 74.2 2.25 95.2 5.8 47 8.7
FLGP0-2 19.8 0.44 78 2.7 96.4 5.95 53 8.32
FLGP30-1 22.2 0.45 81.8 2.85 105.3 6.5 51 10.8
FLGP30-2 20.2 0.55 83.2 3.8 106.2 6.9 63 13.4
FHGP0-1 15.9 0.35 - - 100.2 4.86 58.9 6.02
FHGP0-2 17.3 0.31 - - 99.9 5.11 53 5.9
FHGP30-1 25 0.48 - - 108 5.9 53 7.3
FHGP30-2 29.3 0.45 - - 110.2 5.36 58.87 6.6

Service Stiffness

To ensure functionality and performance of a structure under normal operating conditions, it is essential to determine its service stiffness. This parameter evaluates the ability of the structure to resist deformation under service loads, in comparison to its response under ultimate loads. For that purpose, information such as initial linear response to loading, deflection and cracking under normal operating conditions were obtained. The service stiffness of each specimen is obtained by calculating the slope between the two points corresponding to 50% and 80% of the ultimate load on the ascending branch of the load–deflection curve [46]. The cracked section of the load-deflection profile is only considered for the calculation of service stiffness. Under service loads, concrete is in the cracked state prior to yielding of the reinforcement. Similar ranges in the services stiffness slopes of GP beams and normal beams are observed as shown in Figure 10. Diminished service stiffness values of 16.9% and 21.1% for GP30 under-reinforced beams and 18.5% and 23.3% for GP30 over-reinforced beams than the respective GP0 beams are observed. The marginal decrease in service stiffness is due to the increased deflection of GP beams compared to the normal beams at first crack state as summarised in Table 3.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F10.jpg
Figure 10.

Service stiffness of under and over-reinforced GP beams.

Post-peak Stiffness

Evaluation of a material’s behaviour after it has reached its maximum strength or load-bearing capacity is significant for determining the safety and stability of a structure. A material with high post-peak stiffness can sustain larger deformations before collapse of the structure. The post-peak behaviour can reveal the cause of failure and the material’s resistance to fatigue. The post-peak stiffness for each specimen was obtained by calculating the slope between the points corresponding to 90% and 80% of the ultimate load on the descending branch of the load–deflection curve [46]. Rate of loading was smaller in the post-peak region. Lower post-peak stiffness slopes than service stiffness slopes are observed for all the under-reinforced beams (which is because of yielding of reinforcement) as shown in Figure 11. The reduced post-peak stiffness is attributed to the increased ultimate deflection of the under-reinforced beams. Anyhow, the over-reinforced beams possess a significant increase in the post-peak stiffness than the service stiffness as a result of the reduced ultimate deflection. The evaluation of post-peak slopes shows a slightly declining trend for GP30 beams due to its increased deformation capacity than GP0 beams. However, the post-peak values of GP beams lie in the same range as that of normal beams.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F11.jpg
Figure 11.

Post-peak stiffness of under and over-reinforced GP beams.

Flexural Toughness

Assessing the flexural toughness is important as it gives information about the material’s ability to withstand crack growth and fracture under bending loads. Therefore the flexural toughness of the test beams is evaluated based on the area under the load-deflection curve for a specified deflection value [46]. Greater flexural toughness is observed for GP specimens at an increase of 36% and 66% for under-reinforced beams and an increase of 48% and 12% for over-reinforced beams than the corresponding control beams. The enhanced flexural toughness exhibited by the GP30 beams compared to GP0 beam signifies the higher capacity of the GP30 beam to resist failure under loads. Further the increased load capacities and increased deflection (Table 3) also adds the flexural toughness in GP infused beams than the control beams. The enhanced flexural toughness observed for the under-reinforced beams is due to its higher deflection behaviour under loads than the over-reinforced ones as shown in Figure 12.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F12.jpg
Figure 12.

Flexural toughness of under and over-reinforced GP beams.

Ductility Index

To assess the seismic performance of structures, it is essential to understand the plastic and elastic deformation exhibited by the structural members before fracture, by means of ductility index. Hence in the present study, the displacement ductility index is evaluated by taking the ratio of the vertical displacement in the midspan corresponding to the ultimate load to that of the yield load [47, 48]. Alternatively, it is also represented as ratio of the vertical displacement in the mid-span corresponding to the peak load to that of the yield load. The ductility index is expressed as µ = δu/ δy or µ = δp/ δy. Ultimate load is taken as 80% drop in the peak load and yield load as 65% of the peak load. The calculation of ductility is more realistic since the area under load-deflection curve is greater in the case of four-point bending test.

The present study demonstrated a decrease in ductility index with an increase in the reinforcement ratio as illustrated in Figure 13. The under-reinforced sections indicate only a marginal decrease in ultimate displacement ductility at 2% and 9% for the GP30 specimens compared to the GP0 specimens. However, the over-reinforced GP beams show more significant variation in diminishing the ultimate displacement ductility index at 28% and 34% than the companion control beams [44, 45]. The same trend of decrease in peak displacement ductility for the GP beams has also been observed.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F13.jpg
Figure 13.

Ductility index of under and over-reinforced GP beams.

Longitudinal Strain Profile of Under-reinforced Concrete Beams

Longitudinal strains across the depth of the beam are measured at each load increment using a demountable DEMEC gauge. From the recorded data, horizontal strain distribution up to the peak load is plotted, as shown in Figure 14. The strain distribution demonstrates an almost linear pattern across the beam depth for both GP0 and GP30 beams. It is also observed that there is an increase in tensile strains for both GP0 and GP30 specimens compared to compressive strains due to the stretching of bottom fibres in the tension zone. A decrease in the compressive strains with an increase in the applied load is also noticed. Higher values of strains in the tension zone for each load interval follow the ductile behaviour of the beams due to the yielding of the tension reinforcement. An increase in the load application led to an increase in the tensile strains and thereafter the crack formation. The concrete strains at peak loads for the GP30 beams are higher compared to the GP0 beams. The concrete strains recorded at the midspan of the beam at the first crack load for the GP30 and GP0 beams are 0.003124 and 0.0020074 respectively, which increased with an increment in load. At midspan, the maximum strain of the GP30 specimen is recorded as 0.0147 at a load of 105 kN, which is 25% higher than that of the GP0 specimen with a strain of 0.011 at an ultimate load of 95 kN. Presence of small strains obtained for the DEMEC points at the middle longitudinal layer at the midspan region is due to the slight shift of the neutral axis towards the compressive zone, the typical property of the under-reinforced section.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F14.jpg
Figure 14.

Horizontal strain distribution across the depth of under-reinforced beams.

Moment Curvature Relationship of Under-reinforced Concrete Beams

Formation of cracks leads to the variation of curvature along the length of the beam which arises from the fluctuation of the neutral axis along the length of the beam during loading. The results obtained from strain gauge and deflection measurements are comparable and provide similar values for the curvature [49]. Accordingly, DEMEC gauge buttons are positioned on the lateral surface of the beam to measure the horizontal deflection at the midspan and loading point of the beam. The moment-curvature relationship is determined by analysing the horizontal deflection noted from the DEMEC gauge points at each load level. The high curvature values resulting from the GP beams are attributed to the high deflection experienced by the GP beams during loading.

The curvature produced by the GP beams as shown in Figure 15 is larger compared to the control beams and this is attributed to the greater deformation exhibited by the GP beams. Increased moments of the GP beams are also due to an increase in the curvature.

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F15.jpg
Figure 15.

Moment curvature relationship of under-reinforced beams.

SEM Micrographs of GP Concrete Samples

Figure 16 shows the SEM images of GP concrete test samples at different magnification and the formation of silicates in the GP test sample.

The SEM micrographs shown in Figure 16 illustrate the compactness of the CSH structure on the surface of GP particles, indicating the absence of any large voids or cracks. The strength enhancement of GP concrete (referred to in Table 2) is attributed to the uniform distribution of GP grains and its dense microstructure. The strength of the composite is due to the chemical transformation of hydroxyl groups [Si-OH and Ca(OH)2] formed during cement hydration into strong Si-O-Si covalent bonds during the hardening process. The SEM images (Figure 16(a) and (b)) reveal GP grains surrounded by a CSH rim, demonstrating the strong interfacial bonding. The strength enhancement is also confirmed by the formation of silicates on the surface of GP concrete as shown in Figure 16(c).

https://static.apub.kr/journalsite/sites/durabi/2023-014-04/N0300140401/images/Figure_susb_14_04_01_F16.jpg
Figure 16.

SEM micrographs of GP concrete.

Conclusion

Greener concrete was produced by the partial substitution of cement by GP, which was obtained by mixing equal proportion of two glass powders having different fineness. The research uses a unique mix design with a minimum void concept that incorporates GP of two gradations. On the assessment of mechanical properties of concrete with GP at varied cement substitutions, it was observed that cubes with 30% GP produced highest compressive strength. Therefore, the present investigation considered beams with 30% GP content and normal beams. Further, the study focuses on examining the flexural behaviour of GP and conventional M40 grade concrete beams through experiments on both under and over-reinforced concrete sections. The influence of GP on the flexural performance of concrete beams for a cleaner production is analysed.

•An increase in the first crack, yield and ultimate moments are noticed for the beams with GP concrete for both under and over-reinforced beams with an increase in the reinforcing ratio.

•The ultimate load carrying capacity of beam is improved by the partial replacement of cement by hybrid GP in normal concrete.

•An increase in both load and deflection values are observed in under-reinforced GP beams, while over-reinforced GP beams exhibited higher load values with lower deflections. However, higher loads and deflections are observed for the GP concrete than their counterparts.

•Improved flexural toughness observed for both under and over-reinforced GP beams is due to its higher deflection behaviour under loads than the corresponding control beams.

•Over-reinforced beams possess a significant increase in the post-peak stiffness than the service stiffness as a result of the reduced ultimate deflection. Evaluation of post-peak slopes show a slightly decreasing trend for the GP30 beams due to its increased deformation capacity than the GP0 beams.

•Diminished service stiffness values of 2.1% and 9.8% for the GP30 under-reinforced beams and 28.9% and 34.8% for the GP30 over-reinforced beams than the respective GP0 beams are observed due to the increased deflection of the GP beams than the normal beams.

•A marginal decrease in ultimate displacement ductility of 2% and 9% is observed for under-reinforced GP beams than the companion control beams, while over-reinforced GP beams show a more significant reduction of 28% and 34%.

•An increase in tensile strains for both the GP0 and the GP30 under-reinforced beams are attributed to the high deflections and increased loads experienced by the GP beams during loading.

•The acceleration of cement hydration with the addition of GP is very well understood from the microstructure properties. During hydration, the free calcium content in cement reacts with silica of the GP and forms additional CSH gel due to the formation of strong Si-O-Si covalent bonds on hydration, and thus resulted an increased strength.

Therefore, the researchers are motivated to consider the current disposal of waste glass in India from landfills to its utilisation as partial substitution for cement leading towards a greener production of structural concrete.

Acknowledgements

The authors would like to thank the financial support from the Transportation Research Centre (TRC), CET, Trivandrum, India, under grant no.: TRC 20CET RP2 and the All India Council for Technical Education (AICTE). The microstructural analysis as part of the study were performed at National Centre for Earth Science Studies (NCESS) and Central Laboratory for Instrumentation and Facilitation (CLIF), and it is hereby acknowledged.

Disclosure Statement

The authors declare they have no conflict of interest.

Funding Details

This work was financially supported by The Transportation Research Centre (TRC), College of Engineering Trivandrum, Trivandrum, India under grant no.: TRC 20CET RP2 and The All India Council for Technical Education (AICTE).

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